This invention relates to an improved self-regulating electrical heating element and, in particular, relates to an improved self-regulating electrical heating element comprised of a material which exhibits a positive temperature coefficient of resistance.
A positive temperature coefficient of resistance may be defined as a substantial increase in the electrical resistance of a material with a small increase in the temperature of the material beyond a specified temperature. Compositions exhibiting a positive temperature coefficient of resistance are frequently referred to as PTC compositions. The resistance-temperature dependence of a typical PTC composition can be graphically depicted as follows: ##STR1##
With reference to the diagram, it can be seen that in the first stages of heating PTC composition, the increase in resistance is small. However, at the temperature designated T.sub.s, further heating causes a rapid increase in resistance as indicated by the large increase in the slope of the curve. The temperature T.sub.s is often designated as the switching or anomaly temperature.
Some of the history and theory of PTC materials is discussed by Meyer, "Glass Transition Temperature as a Guide to Selection of Polymers Suitable for PTC Materials," Polymer Engineering and Science, November 1973, Volume 13, No. 6. Other discussions of the characteristics of PTC material are found in Kampe U.S. Pat. No. 3,823,217 issued in July, 1974, Kohler et al U.S. Pat. No. 3,351,882, issued November, 1967, and in an article by Ting, "Self-Regulating PTC Systems," Texas Instruments Publication, 1971 (recommended by IEEE Domestic Appliance Committee for presentation at the 22nd annual Appliance Technical Conference, Chicago, Illinois, May 4 and 5, 1971). The disclosures of these references are incorporated herein.
In recent years, PTC compositions have been employed as components in self-regulating electrical heaters. One prior heating element was formed as a thin strip capable of being wrapped around a substrate. Such element was used to heat tubular or irregular conduits or vessels, for example, to thaw their contents, to prevent the salting out of solids in solution, etc. The element was comprised of PTC materials disposed between two electrodes formed at opposite sides of the strip and running parallel down the length of the strip. An insulating material was disposed around the outside of the strip. In operation, the electrodes were energized and a potential gradient was formed along the plane of the strip transverse to its longitudinal axis. At a constant applied voltage directed across the PTC material, the current (I=V/R) through the heater was large at low temperatures. The power (P) generated by this current (P=I.sup.2 R) was dissipated as joule heat thereby warming the PTC composition. If the applied voltage was high enough, the temperature continued to rise without a significant increase in resistance until the T.sub.s temperature was reached. At this point, a further increase in temperature of the material resulted in a significant increase in resistance. Since the applied voltage was constant, the further increase in temperature resulted in a corresponding decrease in current and therefore in power generation. In effect, the heater was switched off.
In operation, the heat built up in the PTC composition was dissipated by heating its surroundings until its temperature dropped below T.sub.s at which point the power output of the heater again rose. In actual practice, a steady state condition was attained at about the T.sub.s temperature as heat loss to the surrounding was offset by heat being generated within the PTC composition. The net effect was that the power being generated by the current in the PTC composition remained relatively constant as did its heat output. Unfortunately, in some cases, prior art strip PTC heaters have experienced thermal instabilities. Thermal instabilities have resulted in the formation of non-uniform areas of resistance in the PTC material. For example, thermal instability has resulted in the formation of "hotlines" in the heater. The hotline effect is the formation of a narrow band in the PTC material having a higher temperature than the surrounding PTC material. This band has a correspondingly higher resistance than the surrounding cooler PTC material. The band ran generally parallel to and between the electrodes and normally ran down the entire length of the strip. The current flow through the PTC material caused the band to heat up at a faster rate than the surrounding PTC material. Even though the total current flow through the heater decreased as the temperature of the band increased, the increase in the resistance of the band still caused the band to heat up at an even faster rate than the surrounding PTC material. In this way, the band became even hotter and in some cases, the instability grew to a point where almost all of the joule heat generated in the strip came from the band. In extreme cases this caused a burn out of the insulating jacket and failure of the heater.
Prior art solution to the hotline problem involved forming the PTC heaters having generally dumbbell-shaped cross-section with the central layer of PTC material formed as thin as possible to enable efficient uniform heat dissipation normal to the plane of the strip to prevent the hotline effect. Unfortunately, these heaters still experience hotlines.
It has been found that the hotline problem may be mitigated by increasing the relative thickness of the central portion of the PTC material in relation to the distance between the electrodes.